The present application relates generally to the use of liquid desiccants to dehumidify and cool, or heat and humidify an air stream entering a space. More specifically, the application relates to the replacement of conventional mini-split air conditioning units with (membrane based) liquid desiccant air conditioning system to accomplish the same heating and cooling capabilities as those conventional mini-split air conditioners and at the same time to provide additional functionality such as, for example, the ability for the system to heat and simultaneously humidify the space or for the system to heat and simultaneously dehumidify a space thereby providing for healthier indoor air conditions than conventional systems will provide.
Desiccant dehumidification systems—both liquid and solid desiccants—have been used parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that require large amounts of outdoor air or that have large humidity loads inside the building space itself. (ASHRAE 2012 Handbook of HVAC Systems and Equipment, Chapter 24, p. 24.10). Humid climates, such as for example Miami, Fla. require a lot of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Desiccant dehumidification systems—both solid and liquid—have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as ionic solutions of LiCl, LiBr or CaCl2 and water. Such brines are strongly corrosive to metals, even in small quantities, so numerous attempts have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. In recent years efforts have begun to eliminate the risk of desiccant carry-over by employing micro-porous membranes to contain the desiccant solution. These membrane based liquid desiccant systems have been primarily applied to unitary rooftop units for commercial buildings. However, residential and small commercial buildings often use mini-split air conditioners wherein the condenser (together with the compressor and control system) is located outside and the evaporator cooling coil is installed in the room or space than needs to be cooled, and unitary rooftop units are not an appropriate choice for servicing those spaces. In Asia in particular (which is generally hot and humid) the mini-split air conditioning system is the preferred method of cooling (and sometimes heating) a space.
Liquid desiccant systems generally have two separate functions. The conditioning side of the system provides conditioning of air to the required conditions, which are typically set using thermostats or humidistats. The regeneration side of the system provides a reconditioning function of the liquid desiccant so that it can be re-used on the conditioning side. Liquid desiccant is typically pumped or moved between the two sides, and a control system helps to ensure that the liquid desiccant is properly balanced between the two sides as conditions necessitate and that excess heat and moisture are properly dealt with without leading to over-concentrating or under-concentrating of the desiccant.
Mini-split systems typically take in 100% of room air through the evaporator coil and fresh air only reaches the room through ventilation and infiltration from other sources. This often can result in high humidity and cool temperatures in the space since the evaporator coil is not very efficient for removing moisture. Rather, the evaporator coil is better suited for sensible cooling. On days where only a small amount of cooling is required, the building can reach unacceptable levels of humidity since not enough natural heat is available to balance the large amount of sensible cooling. Equally on colder humid days, such as in the rainy season, heating the air would be preferred while also dehumidifying it. Mini-split systems are typically unable to provide dehumidification, although they will provide heating if they are setup as a heat pump.
In many smaller buildings a small evaporator coil is hung high up on a wall or is covered by a painting as for example the LG LAN126HNP Art Cool Picture frame. A condenser with compressor is installed outside and high pressure refrigerant lines connect the two components. Furthermore a drain line for condensate is installed on the indoor coil unit to remove moisture that is condensed on the evaporator coil to the outside. A liquid desiccant system can significantly reduce electricity consumption and can be easier to install without the need for high pressure refrigerant lines. The advantage of such an approach is that a significant portion of the cost of a mini-split system is the actual installation (the running, filling and testing of refrigerant line) that need to be installed on site. Furthermore, since the refrigerant lines run into the space, the refrigerant selections are limited to non-flammable and non-toxic substances. By keeping all of the refrigerant components outside, the number of available refrigerants can be expanded to include ones that otherwise would not be allowed, such as propane etc.
There thus remains a need to provide a retrofittable cooling system for small buildings with high humidity loads, wherein the cooling and dehumidification of indoor air can be accommodated at low capital and energy costs.